Nitride semiconductors are used in the creation of new solid-state lighting, highly efficient amplifiers for wireless communications, advanced power electronics with unprecedentedly low losses, and a large array of new high performance devices, for example.
Among semiconductor materials, gallium nitride (GaN) has been shown to have good electrical conductivity, thermal conductivity and thermal stability. In addition, due to its wide band-gap, GaN is capable of emitting at the green to violet wavelength and is also suitable as a full-color light-emitting element. However, the typical substrates, such as silicon substrates, used for epitaxial growth of GaN have huge differences in lattice constant and thermal expansion coefficient from GaN, namely 17% and 46%, respectively; leading to large stresses during and after their production. As a result of these excessive tensile stresses, bending/bowing or cracking of the heterostructure may occur during the cooling process after the completion of GaN epitaxial film growth, resulting in reduced component yield. For example, FIG. 1 illustrates a cross-section of a heterostructure 10 resulting from conventional GaN epitaxial growth on a silicon substrate. As shown, due to the differences in lattice constant and thermal expansion coefficient the formed heterostructure may exhibit a bending/bowing with the GaN layer having a concave primary surface 12. Adopting a sapphire (Al2O3) substrate may have the same problem (16% lattice mismatch and 34% differences in thermal expansion coefficient) along with a higher cost. In addition, silicon carbide (SiC) can also be used as the substrate (3.5% lattice mismatch and 25% differences in thermal expansion coefficient). However, the cost of SiC is too high for routine use in commercial purposes.